USB is a technology which provides a fast, cabled data connection between a complex device (e.g. a PC) which is called the Host and a connected peripheral (e.g. a mouse, keyboard etc.) which is called the Device.
The USB standard has been extended to include connections between mobile devices in the USB on-The-Go (OTG) supplement. This supplement allows a peripheral at either end of the connection to take a Host or Device role. The initial roles are decided by the direction of the cable connection i.e. the type of plug A or B inserted into the peripheral.
The peripheral with the USB OTG A-plug inserted is called the A-Device, wherein this A-Device supplies power for the duration of the connection and will initially take the host role. The peripheral with the USB OTG B-plug inserted is called the B-Device, wherein the B-Device draws power from the A-Device and initially takes a device role.
Since USB provides the capability to provide power from one device to another it is also possible to make a charger which utilises the USB port. Such a charger is currently defined in the white paper “USB charging white paper”, Revision 0.81, Jul. 22, 2006 by the USB implementer's forum. Detection of such a USB charger requires that the charger outputs a logical high on the D− line when the D+ line is pulled high and the D− line is pulled low.
Many peripherals are provided with one single USB connector only. This shows the problem of charging a USB unit, e.g. by means of the above-mentioned charger, and simultaneously providing data passthrough to another USB device via a single USB connector of said USB unit, and thereby not effecting OTG functionalities.